Often in wireless communications interfering signals share the same frequency band (or channel within the band) as the desired signal. As noted above, the interfering signal can be the desired signal arriving along a reflected path or paths. This will be referred to as coherent interference, which can lead to partial cancellation of the signal strength. This in turn can result in signal fade or dropout.
An independent interfering signal will be referred to as incoherent interference. This type of interference is often characterized as either broadband or narrow band interference. Broadband interference is spread over a large fraction of or all of the bandwidth associated with the desired signal. This interference looks like noise to the system and will effectively reduce the signal to noise ratio (SNR) and can swamp the desired signal or at least reduce its quality. Narrowband interference occupies a smaller fraction of the signal band. Applying narrowband-filtering or narrowband-processing techniques to the antenna output can sometimes mitigate its deleterious effect.
Interference may unintentionally compete with the desired signal, as is the case in an area where two co-channel radio stations have about the same strength. In some situations (warfare) intentional interference can occur. Sometimes the interfering signal has been intentionally modulated so as to mimic some key aspect of the desired signal. This can corrupt the information content that the receiver outputs. For digital communications both coherent and incoherent interference can lead to unacceptable bit error rates, loss of signal lock, or a corruption of the information or message in the desired signal.
The conventional method of designing a wireless system for interference rejection is to receive outputs from two or more antenna elements. A processor uses these outputs to determine a complex weight or set of weights for each output. These are applied to the measured outputs to produce weighted outputs. These weighted outputs are then combined to form a single output. If the weights are chosen correctly, the effective power of the interference in the final output will be significantly reduced relative to the measured outputs and the desired signal strength will be enhanced. The resulting antenna system is often referred to as an adaptive phased array. If the adaptive array has only a few elements (at least 2 but no more than about 10), then it is often referred to as a “smart antenna.” Actually, the upper bound on the number of elements in “smart” antennas simply reflects current practices and conventions of terminology. In principle this number could be arbitrarily large.
A number of smart antenna systems for communication applications have been described. The “smarts” in such systems make use of a digital signal processor. The inputs to such a processor are the received element signals after the initial front end filtering and down conversion. The processor determines a set of weights that are used to combine the element signals in such a way so as to reduce the interference in the final output. This approach to interference mitigation is performed solely within an electronic package that has two or more antenna input ports. Each such port is connected to an antenna element via an RF (radio or carrier frequency) transmission line of some type. The antenna elements are designed to have coverage that is as broad as possible but are offset from each other in position and/or orientation. These offsets have to be large enough so that there are sufficient signal phase differences among the individual element outputs. The processor uses these phase differences to advantage in determining the appropriate weights. For adequate spatial filtering element separations ranging from 0.3 to 0.5 carrier wavelength are required.
A number of U.S. patents disclose variations on the theme of the type of smart antenna described above. U.S. Pat. No. 6,122,260 discloses a smart antenna system for CDMA wireless applications. This system uses multiple antenna elements and transceivers as well as a processor that exploits spatial and code diversity. U.S. Pat. No. 6,137,785 discloses a smart antenna system for a wireless mobile station. It makes use of at least two antenna elements and a receiver structure for canceling co-channel interference. U.S. Pat. No. 6,177,906 discloses a multimode iterative adaptive smart antenna processing method and apparatus that makes use of multiple antennas and receiver units. A new method for weight selection is also disclosed. U.S. Pat. No. 6,229,486 discloses a subscriber based smart antenna, which uses the outputs from multiple elements to form multiple beams. A controller picks the best beam at any particular time. U.S. Pat. No. 6,252,548 discloses a transceiver arrangement for a smart antenna system in a mobile communication base station. Again, this system uses multiple elements, multiple transceivers, digitizers, and a digital processor. U.S. Pat. No. 6,369,757 discloses a method for a multi-element smart antenna system.
For many of the systems classified as “smart” antennas the total antenna aperture (containing several elements) tends to be a minimum of 1 to 2 wavelengths across. Often the aperture needs to be much larger than this. The elements are typically passive (have fixed properties) and all the interference mitigation is provided at the level of the down converted signal within the system electronics package. Thus, the RF or front end of the system is not affected by the interference mitigating functions of the “smart” antenna system. Typically the elements are designed so that they operate best at a specific carrier frequency as well as across a fairly narrow band (a few per cent relative bandwidth) about that frequency. Dual tuned elements also exist and could possibly be used for “smart” antenna applications.
Conventional “smart” antenna systems can be very effective in mitigating the impact of one or several interfering sources. However, they also have significant drawbacks. Among the most significant ones are:                1. Multiple antenna outputs must be handled simultaneously. This means multiple matching networks, filters, and down-converters and possibly multiple LNAs at the front end. For some applications, the system will also require multiple AD converters.        2. The required total antenna aperture may be unacceptably large for many applications. Such apertures will range from 1 to 2 wavelengths to several wavelengths across.        3. Typically the system will be restricted to a fairly narrow range of carrier frequencies. This limitation occurs at the RF front end. The down converting electronics could be designed to provide down conversion over a wide range of frequencies, and the rest of the electronic package (including the processor) is limited by the bandwidth and is basically unaffected by the carrier frequency.        
A number of U.S. patents disclose variations on antenna system designs that make use of parasitic elements. A number of these specifically describe arrays of parasitics within multi-element arrays of active elements. Examples are as follows. U.S. Pat. No. 5,294,939 discloses a multi-element reconfigurable antenna system that uses microstrip patch elements—both active and parasitic. The parasitic element(s) could be passive or loaded with variable impedances. The emphasis is on array applications where the overall system size would be at least a few wavelengths. U.S. Pat. No. 6,040,803 discloses a multi-element antenna system that makes use of passive parasitics to provide dual band capabilities. U.S. Pat. No. 6,317,100 discloses a planar antenna array with passive parasitic elements to provide multiple beams of varying widths. In this system a single active element is used for transmitting and multiple elements are used for receiving.
A number of single element designs with passive parasitics are also disclosed in the prior art. Examples are as follows. U.S. Pat. No. 5,923,305 discloses a dual band helix with a second passive parasitic helix that is either collocated with or adjacent to the active element. The presence of the parasitic enables the antenna element to be tuned at two different bands. U.S. Pat. No. 6,133,882 discloses an antenna element that uses parasitics for parasitic feed coupling to a radiating element. U.S. Pat. No. 6,181,279 discloses a patch antenna element with an electrically small ground plane. Peripheral parasitic slabs are used to help tune the antenna assembly to a desirable frequency. U.S. Pat. No. 6,198,943 discloses the use of a passive parasitic for dual band tuning of an internal loop dipole antenna. U.S. Pat. No. 6,249,255 discloses an antenna assembly and associated method that makes use of a passive parasitic to reduce the gain in the direction of the user of a communication device. U.S. Pat. No. 6,285,327 discloses a substrate antenna element that makes use of a passive patch parasitic to tailor the antenna characteristics.
In “Axial Mode Helical Antennas” Nakano et al. describe the use of a passive helical parasitic element with an active helical element. The parasitic element is shown to have a noteworthy impact on the element gain pattern. In “A Planar Version of a 4.0 GHz Reactively Steered Adaptive Array” Dinger describes a planar array that includes a single active microstrip element and eight closely coupled parasitic microstrip elements that are reactively loaded with variable impedances. The parasitic elements are exterior to the aperture of the active radiating element. The dimensions of the array are about 1.0×1.5 wavelengths. Null steering for the active element at 4.0 GHz is demonstrated for the active element.